Technology for forming a semiconductor film on a semiconductor substrate by epitaxial growth has been used widely and conventionally in forming various semiconductor elements. For example, technology for forming a crystal of silicon, silicon germanium, or the like on a silicon substrate has been studied vigorously in recent years as technology which enables the formation of a high-performance device such as a hetero bipolar transistor. The silicon-based epitaxial growth can be effected by, e.g., UHV-CVD whereby a gas source is supplied to a surface of a semiconductor substrate that has been heated in a vacuum vessel or MBE whereby a molecular beam obtained by heating a solid source is supplied to a semiconductor substrate.
In either of the aforesaid crystal growth methods, if an impurity exists on the surface of the substrate during the epitaxial growth, the impurity forms a nucleus to cause a crystal defect, which seriously impairs the quality of a crystal layer grown. In initiating the crystal growth, therefore, the semiconductor substrate should have an extremely clean surface. Such a clean surface can be obtained by cleaning a semiconductor substrate, forming a thin (on the order of 10 to 20 .ANG.) chemical oxide film which does not contain an impurity such as carbon on the semiconductor substrate by chemical treatment, and then removing the oxide film in the vacuum vessel.
The protective oxide film method for providing the clean surface using the chemical oxide is disclosed in detail in a paper written by Ishizaka et al. (1986 Journal of Electrochemical Society p. 666) and in a paper written by Tatsumi et al. (1985 Japanese Journal of Applied Physics Vol. 24, No.4L227).
Since a chemical oxide itself may cause a crystal defect, it should be removed completely at the initiation of crystal growth. To remove the chemical oxide, however, a heating process should be performed at a temperature (about 750 to 850.degree. C.) higher than a temperature (about 450 to 600.degree. C.) during the crystal growth.
If a heat treatment is performed with respect to a substrate that has been patterned for device fabrication, on the other hand, the problem of a change in impurity profile or pattern deformation occurs. To provide reliable device properties, therefore, the substrate is preferably heated to a lowest possible temperature and for a shortest possible period. Since the process of heating the chemical oxide is performed at the high temperature as described above, it will be understood that performing the heating process at a lower temperature and for a shorter period is particularly important in providing reliable device properties.
In the heating process for removing the chemical oxide, therefore, an attempt has been made conventionally to perform the heating process at a lower temperature and for a shorter period by making the following observation.
Lowering of Temperature of Reaction for Removing Oxide Film by Using Si-Containing Gas and Conventional Method for Observation
Although SiO.sub.2 as a main component of the chemical oxide is by itself an eminently stable material, Si composing the semiconductor substrate reacts with SiO.sub.2 at the interface between the semiconductor substrate and the oxide film to generate SiO. Because SiO has high volatility, SiO.sub.2 is lost gradually as SiO is generated to evaporate.
To accelerate the reaction for removing the protective oxide film, therefore, it is effective in causing a reaction for forming SiO at a surface portion of the semiconductor substrate, while supplying a gas containing Si. It has been reported by a paper written by M.Hirayama et al. (1987 Applied Physics Letters, No.51, p.2213) that the time required to remove an oxide film can be reduced from conventional 10 minutes at 840.degree. C. to 2 minutes at the same temperature of 840.degree. C. by introducing silane (SiH.sub.4) into a vacuum vessel during the process of removing the oxide film. If the time required to remove the oxide film is maintained at the same 10 minutes, the temperature can be lowered to 815.degree. C.
A silicon-containing gas such as silane may be supplied as it is, as disclosed in Japanese Unexamined Patent Publication No. SHO 64-4026. It is also known that the silicon-containing gas such as silane is effective if it is supplied after being cracked, as disclosed in Japanese Unexamined Patent Publication No. HEI 1-305893.
The paper and publications disclose the removal of the oxide film which is performed under observation by reflection high-energy electron diffraction (RHEED).
Problems to be Solved
However, RHEED used in the processes disclosed in the paper and publications necessitates measurement made in a vacuum chamber, so that it is impossible to make a close in-situ observation of a state change during the removal of an oxide film conducted by using an etching gas, though it is possible to detect the state in which the oxide film has been removed completely. Accordingly, it is difficult to properly change conditions for the heating process as the removal of the oxide film proceeds.
In accordance with the processing methods disclosed in the paper and publications, therefore, the oxide film is removed based on the conditions determined empirically and an optimum method of removing an oxide film responsive to a change in the state of the oxide film during the removal process has not been established yet. In particular, the mechanism of removing the oxide film has been unknown.
It is therefore an object of the present invention to provide means for making in-situ observation of the process of removing a protective oxide film and thereby providing a method of fabricating a semiconductor device and a fabrication apparatus therefor which are for properly removing the oxide film as the removal of the oxide film proceeds.